InstSimplify: Optimize signed icmp of -(zext V)

Summary:
We know that -(zext V) will always be <= zero, simplify signed icmps
that have these.

Uncovered using http://www.cs.utah.edu/~regehr/souper/

Reviewers: nicholas

Subscribers: llvm-commits

Differential Revision: http://reviews.llvm.org/D3754